Resistors of polycrystalline silicon, also called polysilicon, have been used within the electronic field during more than thirty years. Methods of manufacturing polycrystalline silicon are known as well as methods of manufacturing resistors from polycrystalline silicon. It is also known how the resistivity of the polysilicon material can be controlled to obtain a desired value by adding dopants to the material. The general technique is illustrated in the book "Polycrystalline Silicon for Integrated Circuit Applications" by T. Kamins, ISBN 0-89838-259-9, Kluver Academic Publishers, 1988.
In analogue electronic circuits the requirements of stability of resistors included in the circuits are extremely high: the specifications in regard of the maximum allowed change of the absolute value of the resistance must be fulfilled for a resistor in such a circuit all the time when the circuit is used and the resistance changes of resistors which are matched to each other must be such that the ratio of the resistances to each other is maintained all the time when the circuit is used. These requirements thus comprise that the resistors must be sufficiently stabile during all the time when the circuit is used.
In applications where polysilicon resistors are used in critical parts of electronic circuits the insufficient stability of such resistors is a known practical problem. The fact is that the resistors can change their resistance values in an unpredictable way during the use thereof. Such deviations from the value determined by the designer as well as deviations of the resistance values of matched resistors can jeopardize the operation of the electronic circuit in which such resistors are included. The cause of the instability is to be searched in the unsaturated bonds which exist in the grain boundaries of the polysilicon material. The unsaturated bonds are formed in the boundaries between the individual monocrystalline grains in the polycrystalline silicon material owing to the fact that the periodic order of the silicon atoms in their crystal lattice inside the grains does not exist at the boundaries. The outermost silicon atoms in a crystalline grain therefore have not sufficiently many silicon atoms as their closest neighbours in order to form the four bonds which are characteristic of the lattice of silicon crystals. The resulting unsaturated bonds at the grain boundaries act as traps of charge carriers and bind thereby electrical charges to the grain boundaries, influencing the capability of the material of transporting charge carriers and thereby the resistivity of the material.
If the number of bound charges would remain constant from the instance when the manufacture of the resistors was finished and during all time when the resistor was used there would not exist any problems relating to the stability of the resistors. However, the number of traps can decrease if individual atoms can migrate out of the grains, into the grain boundaries and there be attached to the unsaturated bonds and thereby prevent that the latter ones can continue to work as traps of the charge carriers. In the similar way the number of traps can increase in the case where atoms leave their positions at the grain boundary and thereby leaves an unsaturated bond behind.
It is known that the unsaturated bonds can be blocked by adding hydrogen atoms to the grain boundaries. Hydrogen can exist in a rich amount in layers deposited on an integrated circuit containing a polysilicon resistor, e.g. in passivating layers of silicon dioxide or silicon nitride, such as typically 20-25% in the passivating films of silicon nitride which are produced by plasma CVD and which conventionally are used as a protection for finished integrated circuits and components.
The hydrogen atoms react with the unsaturated bonds and block them so that they cannot continue to operate as traps. However, a problem related to hydrogen atoms which have been bound to unsaturated bonds, is that the binding strength between hydrogen and silicon is low compared to e.g. the bond of silicon to silicon. The bonds can therefore easily be broken and then the hydrogen atoms diffuse away from their positions at the grain boundaries and the unsaturated bonds are again exposed. Since unsaturated bonds captures charge carriers, it will result in that the resistance value is changed. The causes of the fact that the bonds are broken are not completely known, but they can be related to a general increase of temperature or to local temperature variations caused by increased power generation in critical points of the resistor. However, it cannot be excluded that the bonds can also be broken owing to purely kinetic effects caused by running charge carriers.
Even if the capability of the hydrogen atoms of blocking unsaturated bonds is what is primarily discussed in literature, it cannot be excluded, that other atoms which happen to be placed in a grain boundary or leave it produce similar effects, if they cannot bind sufficiently strongly to the silicon atoms at the grain boundary. Without indicating here the magnitude of the influence, it can be mentioned that it also is possible that dopant atoms which during the use of the resistor interact with the grain boundaries in a dynamic way, can have the same influence on the resistivity as the hydrogen atoms. In the same way it cannot be excluded that also other atom kinds included in the resistor and unintentionally added impurities can have the same influence.
A stabilized resistor built on polysilicon is disclosed in the published International patent application WO 97/10606. The material of the resistor body is doped with both acceptors and donors. In order to block charge carrier traps in grain boundaries to a sufficient degree and thereby confer a good stability to the resistor, when it is exposed to different substances in the sequence of technical processing steps in the manufacturing procedure, and also a good long-time stability, the doping with donor atoms is made in such a high concentration that if only the donor atoms would be present in the material and substantially no acceptor atoms, the material would be considered as more or less hard doped. It means rather high concentrations of doping atoms and as has been indicated above these atoms can to some extent move into and out through the grain boundaries. It is due to the fact that the segregating mechanism which causes dopant atoms to be placed in ground boundaries in the heat treatments or annealing operations, is active also at lower temperatures, though to a much smaller extent. Such resistors stabilized by means of compensation doping could therefore be less stabile.
A compensation doping requires the addition of at least two dopants, i.e. of at least one donor and at least one acceptor, in accurately balanced, high concentrations and there can be difficulties to achieve this in the processing.
A more efficient and stabile blocking of unsaturated bonds than what can achieved by means of hydrogen atoms can be produced by adding atoms of some other suitable kind which form a sufficiently strong bond to the silicon atoms in the silicon grain boundaries. Thus, in U.S. Pat. No. 5,212,108 for Lieu et al. a method of producing polysilicon resistors is disclosed where the resistors are intended to be used in memory cells. In a polysilicon film arsenic ions are implanted in order to define the resistivity of the material. An implantation made thereafter of fluorine ions stabilizes the grain boundaries. Thereby, the variations of the barrier height of the charge carriers between different batches of produced circuit chips are reduced. After implanting fluorine an annealing at 900.degree. C. can be performed.
However, polysilicon resistors manufactured in this way generally appear to have an insufficient long-time stability.
The influence of hydrogen and fluorine on the grain boundaries in polysilicon films has been studied in conjunction with electric characteristics of thin film transistors and solar cells, see e.g. S. Maegawa, T. Ipposhi, S. Maeda, H. Nishimura, T. Ichiki, M. Ashida, O. Tanina, Y. Inoue, T. Nishimura and N. Tsubouchi, "Performance and Reliability Improvements in Poly-Si TFTs by Fluorine Implantation into Gate Poly-Si", IEEE Trans. Electron Devices, Vol. 42, pp. 1106-1112 (1995) and A. Yoshida, M. Kitagawa, F. Tojo, N. Egashira, K. Nakagawa, T. Izumi and T. Hirao, "Hydrogen, fluorine ion implantation effects on poly-crystalline silicon grain boundaries", Solar Energy Materials and Solar Cells, Vol. 34, pp. 211-217 (1994).